The preparation of fine particle systems such as solid-in-liquid dispersions, oil-in-water emulsions, and water-in-oil emulsions can be carried out by a wide variety of processes, including grinding, homogenization, and precipitation. Emulsions are typically described as liquid-in-liquid or amorphous particle-in-liquid systems and are usually prepared by incorporating a liquid or resinous dispersed phase into a liquid continuous phase under high shear mixing or homogenization. Certain emulsions exhibit poor stability due to the surface energetics of the liquid dispersed Phase and may coalesce, crystallize, or degrade in time. Also, most conventional mixers or homogenizers are limited in the ability to reduce particle size below 300 nm.
In certain applications, such as with photographic dispersions, crystalline materials such as dye forming couplers, oxidized developer scavengers, and various dyes are dissolved in organic solvents at high temperatures and emulsified in aqueous gelatin solutions. Submicron amorphous particles in such dispersions are found to be metastable and will eventually recrystallize in this aqueous system unless coated and dried on photographic support, in which state they are stable against recrystallization. Recrystallization of the dispersed particles prior to coating reduces dispersion efficacy and is generally considered undesirable.
The composition of the dispersed amorphous phase is often modified by incorporation of mixtures of solvents and crystalline organic compounds to improve stability against recrystallization. Such additives are often undesirable and may adversely affect photographic response and physical quality of photographic materials.
U.S. Pat. No. 4,865,957, columns 19-21, illustrates the techniques of UV absorber particle formation by melting the UV absorber and incorporating it into a high boiling point organic solvent which is then homogenized and cooled. However, particles formed by such emulsification techniques have been particularly susceptible to recrystallization and crystal growth after cooling. Such crystallization is disadvantageous to product quality due to light scattering and filter plugging during manufacturing by the large grown crystals.
While the solid UV stabilizers have been successful, they are difficult to maintain in the amorphous phase which is preferred to prevent light scattering. Further, the solvent emulsification technique has been expensive and difficult to control. Crystallization of the UV absorber may also lead to delamination of layers, haze, reduced maximum density, stain, and sensimetric problems.
There is a need for a process to overcome prior problems of incorporation of UV absorbers that are solid at room temperatures and in the amorphous phase. Another disadvantage of the present process to be overcome is that the particles of UV absorbers are generally larger than is desirable, thereby resulting in a lesser amount of the absorption, as well as causing more light scattering. It would be desirable if UV absorbers could be made in finer particles with less tendency for crystallization.